A number of applications of ultraviolet photodetectors require devices having both high responsivity and relatively high speed. Such applications include, for example, ultraviolet energy-based optical communication systems and missile warning systems, a list which is considered exemplary but not limiting. The periodic table group III-Nitride alloy, Al.sub.x Ga.sub.1-x N, is a promising material for fabricating such ultraviolet photodetectors partly because of its direct bandgap, which spans the range of 3.4 electron volts for GaN to 6.1 electron volts for AlN. (Such properties are described, for example, in S. Strite and H. Morkoc, "GaN, AlN, and InN: A Review", J. Vac. Sci. Technol. B10, 1237, 1992.) Hence, in view of the direct relationship between bandgap and cut-off wavelength, as is known in the art, photodetectors based on an appropriate choice of III-Nitride alloy composition may provide ultraviolet responses in the wavelength range below a wavelength residing somewhere between 365 nanometers and 200 nanometers, i.e., provide photodetectors of cut-off wavelengths in the range of 200 nanometers to 365 nanometers. These III-Nitride alloys have been useful in the fabrication of ultraviolet light-emitting devices having output in the same spectral range.
The good rejection of long wavelength light thus provided in a III-Nitride ultraviolet detector is additionally desirable for applications exposing the detector to background sunlight "noise". Sunlight, even on the earth's surface, has a strong ultraviolet spectral component in the wavelength range greater than about 300 nanometers in addition to well known visible and infrared components. Ultraviolet photodetectors that do not respond to light of wavelength longer than about 300 nanometers may therefore be referred to as "solar blind" ultraviolet detectors. Photoconductors and phototransistors fabricated from aluminum gallium nitride (Al.sub.x Ga.sub.1-x N) are therefore deemed good candidates for high responsivity, solar blind detectors. The signal gain of some possible detector configurations of this material, such as photoconductor and phototransistor devices, however, is achieved at the expense of device operating speed. Photoconductors in particular have shown extremely long recovery times--as is disclosed in M. Razeghi and A. Rogalski, "Semiconductor UV Detectors", J. Appl. Phys. 79, 7433, 1996 and in B. Goldenberg, J. D. Zook and R. J. Ulmer, "Fabrication and Performance of GaN Detectors", Proc. of the Topical Workshop on III-V Nitrides, Nagoya, Japan, 1995.[2,3].
In contrast with such photoconductors and phototransistors, III-Nitride photodiodes are capable of high speed operation; see, for example, J. M. Van Hove, R. Hickman, J. J. Klaassen, P. P. Chow, and P. P. Ruden, "Ultraviolet-Sensitive, Visible-Blind GaN Photodiodes Fabricated by Molecular Beam Epitaxy", Appl. Phys. Lett. 70, 2282, 1997; and S. Krishnankutty, W. Yang, T. Nohava and P. P. Ruden, "Fabrication and Characterization of GaN/AlGaN UV Band Heterojunction Photodiodes", MRS Nitride Internet Journal, Volume 3, Article 7, 1998. Unfortunately, however, photodiode responsivity, as is disclosed in these same publications, is rather low in view of there being no signal gain mechanism operating in a conventional photodiode. The present invention provides improvement in this gain aspect of a photodiode and makes the III-Nitride photodiode a viable tool for wide bandwidth, solar blind, photodetection.